In 1972 the Clean Water Act was established which provided funding for the discovery and elimination of infiltration in public sewer systems. In general infiltration is the influx of material into a sewer system from unwanted sources. The problem with infiltration arises primarily when groundwater or material from some other source enters sanitary sewer systems through cracks or leaks or through simple permeation through porous walls. Methods used to address the problem of infiltration were primarily the enlargement of treatment facilities in combination with periodic and haphazard repair of cracks, leaks or other infirmities in the traditional, gravity-driven sewer collection system. The enlargement of treatment facilities and storage facilities has and continues to cost local, state, and federal governments billions of dollars. To date, this type of repair and accommodation of extraneous water infiltration has been inadequate to address the continued problem of infiltration. In particular, the building, operation, and maintenance of larger treatment facilities to accommodate greater volumes of infiltration is a cost prohibitive or ineffective use of valuable resources. Further, the expansion of collection systems and enlargement of treatment facilities, both of which comprise enhancements to system capacity, increase the ability of the system to drain a region and, in fact, allow greater amounts of infiltration. Therefore, the solution provided to date has, in fact related to the increased drainage of treatment systems, increased treatment, and an increased volume of infiltration. Finally, if the haphazard repair of sewer systems is practiced, a great deal of time and labor is consumed to monitor, track and discover the location of infiltration of migrating groundwater in different portions of a gravity driven sanitary sewer system.
To compound the problems that existed prior to 1972, the sanitary sewer systems continue to age. Over time, cracks and fissures develop in sewer lines as the ambient groundwater conditions surrounding the sewers vary and sections of sewer are subjected to varying buoyant forces. These buoyant forces typically act to push the sewers upward. Depending on the status of surrounding soil, rock, sand, or other support media, the sewer may or may not be adequately secured to resist these buoyant forces. As years pass and sewers age, the process of repeated infiltration through cracks, fissures, or porous pipes causes flowing water to disturb the pipe-surrounding bed or media. This disturbance may further destabilize the pipe bedding and enhance or accelerate pipe movement due to buoyant forces, thus enhancing deterioration, cracking, and the concomitant increased volume of water entering the system through infiltration.
In general, the problems of infiltration referred to above may be attributed to two causes. The first cause, of course, as referenced above, is the presence of cracks, fractures, leaks, or other infirmities with the aging sewer systems. These systems, which typically are gravity driven, often are partially filled with water and/or a mixture of solids and waste liquid. Typically the system has a head space that is filled with air (or with a mixture of air and gases generated through the anaerobic decomposition of the waste being carried in the pipes). The second factor which leads to the problem of infiltration is the pressure differential between the interior of the sanitary sewer collection system piping and the groundwater which exists on the exterior of the pipe in the pore spaces of the surrounding soil and pipe bedding material. This differential drives the infiltration of groundwater. The pressure in the pipes typically is at a level approximately equal to atmospheric pressure. By contrast, the pressure of the pore water in the soil or bedding surrounding the pipe is often many pounds per square foot (also expressed in psi, pounds per square inch, or referred to as xe2x80x9cheadxe2x80x9d and measured in feet of water). For example, if a sanitary sewer pipe were buried eight feet below ground, and the soil surrounding and lying above the pipe were completely saturated with water all the way to the surface of the ground (as may occur in isolated locations or more widely depending on geographic region or timing relative to rainfall events), the pressure at the 8xe2x80x2 level on the exterior of the sanitary sewer collection pipe would be 8xe2x80x2 of head or approximately 3.5 lbs. per square inch above atmospheric pressure. Such a pressure differential across the conduit wall, when combined with the presence of cracks in the sanitary sewer collection system, leads to infiltrationxe2x80x94the driving of ground water into the collection system through the wall or the cracks.
Therefore, there has been and remains a need to minimize or prevent the infiltration of ground water into sanitary sewer collection systems and the exorbitant cost of storage and treatment of all the extraneous water entering the system. In particular, there is a need for a sewer surcharge system wherein liquid (typically water and dilute waste) may be retained in the sewer collection system on a controlled basis to increase the weight of the collection system and minimize the impact of upward hydrostatic forces which attempt to lift or float the system. Further, the maintenance of a dilute wastewater or fluid within the collection system is needed to minimize the pressure differential across the sewer pipe wall and decrease the tendency for groundwater to infiltrate or flow into the pipe system.
The present invention provides for the surcharging or backing up of a gravity pipe system to a level that will protect the lowest entry ports, such as basements or low lying residences serviced by the gravity sewer system, and which will retain a beneficial level of surcharge material within the system to enhance the weight of the system and minimize infiltration. The surcharging of the pipe lines helps to resist the hydrostatic effect caused by groundwater acting on empty pipe lines wherein the pipe line is subjected to xe2x80x9cfloatationxe2x80x9d due to buoyant forces. The surcharging further serves to at least partially offset the pressure differential across the pipe wall, thus minimizing the driving force for the infiltration and preserving the structural integrity of existing sewer systems.
The surcharged pipelines are controlled by a series of flow-retarding or surcharge maintaining devices such as valves (e.g. pneumatic or hydraulic pinch valves, gate valves, and other valves), sluice gates, dams, weirs or other mechanical or pneumatic means. By allowing the use of flow retarding devices that may be remotely monitored and controlled by a system of computer controlled electronic sensors, a desired degree of automation may be introduced into the system. Preferably, the monitoring system monitors not only pipeline flow, but rainfall and pipeline flow reversal or back-up. Once a rainfall or flow increase is detected, the pinch valve or other flow retarding means may be actuated to close and cause a surcharging of the pipe until such point that the surcharged pipe system is filled all of the way to within a short distance of the bottom of the lowest entry point. Of course, in areas where infiltration present serious problems even in the absence of a rainfall event (and the lengthy period following such an event when water levels recede) the surcharge may be maintained on a more permanent basis as needed and occasionally released to prevent the buildup of organic material and the beginning of anaerobic conditions. For example, gates may be maintained in select locations such as manholes to retain a surcharge and allow flow-over at an elevation just below the lowest critical point.
The surcharge system may be installed throughout an entire collection system or a part of a system with pinch valves or other flow retarding devices and monitoring devices installed as necessary to accommodate elevation differences and protect critical or low areas as needed. Critical areas refer to those areas where surcharged or backed-up water or waste is likely to exit the sewer network into a source (i.e. a basement, a tub, a sink, a toilet, etc.). Critical areas also refer to other areas that may be monitored in order to determine whether an exit is likely or imminent in different location. For example, a given flow level at a selected manhole may provide information sufficient to determine if contamination of source is occurring, likely to occur, or imminent. The throttling effect provided by the pinch valves or other flow retarding means is allowed to continue until such time that peak inflow subsides and a sufficient period of time is allowed to pass following the rainfall event to allow water levels subside to a baseline or dry weather flow level.
The level of automation included in the system may range from a fully manual system to a few selected degrees to a completely automatically functioning system. Additionally, the system of the present invention is preferably designed such that the failure or default position for the system of flow retarding or surcharging devices is an open or wide-open status. In this manner, surcharge liquid is allowed to exit the collection system in the event that one or more of the surcharging devices fail. It is, of course, preferred to provide a manual release for use if system failure results in a valve or gate in a closed position.